1. Field of the Invention
The present invention relates to a conversion lens system and more particularly to a conversion lens system to be attached to an exchangeable objective lens system of a camera at the image side thereof for converting the focal length of the objective lens system into a greater value.
2. Description of the Prior Art
In a conversion lens system of a type which converts the focal length of an original lens system into a greater value, there exists a general problem in that various aberrations of the original lens system are also apt to be converted into greater values when the converter lens system is attached to the original lens system. Further, a remarkable astigmatic difference generally occurs to result in a poor image at the marginal area of the focal plane since the Petzval sum of the original objective lens system with the conversion lens system attached to changed into a considerable negative value because of the negative refractive power of the conversion lens system. There is, accordingly, a demand in the prior art to provide a conversion lens system improved with respect to the aberration problems.
By the way, Japanese Patent Application laid open to the public inspection under No. 52-109930, which corresponds to U.S. Pat. No. 4,154,508, is cited in that it disclosed conversion lens systems having seven single lens elements. In each of the drawings of the Japanese Patent Application, the rear side lens elements beginning from surface r.sub.1 constitute a conversion lens system. Some of the lens elements in the conversion lens system are cemented to each other. Further, the specification of the Japanese Patent Application says that the conversion lens system comprises a negative front lens group consisting of a first positive sub-group, a second negative sub-group and a third positive sub-group, and a positive rear lens group consisting of a fourth positive sub-group and a fifth negative sub-group.